As the density of air traffic has increased, it has become necessary to provide a supervising authority to control the altitude, heading and speed of aircraft, especially in terminal areas. Of course, the supervising authority, to intelligently control any particular aircraft, must have knowledge of other aircraft in the vicinity of the controlled aircraft. To provide the supervisor with such knowledge, various target locating systems have been employed. One such system which has come into widespread use is based upon an interrogator-transponder operation. Using this apparatus, an interrogation signal is transmitted at periodic intervals. Each target (aircraft) to be located carries a transponder and responds to the interrogation pulse by transmitting a particularly identifiable responsive signal. Associated with the supervising authority is a target locating system which includes an antenna scanning 360.degree. in azimuth. When a reply is received, the location of the replying aircraft is determined from two quantities: the azimuth from which the reply signal is received, and the delay between interrogation and reply. Systems of this type are in widespread use and, in unperturbed situations, operate quite effectively.
It has been found, however, that operation of this apparatus in the vicinity of urban areas leads to the presence of false or spurious replies, that is, replies which, in fact, do not relate to the presence of an actual aircraft or at least replies which do not indicate true target location. These false replies have generally been attributed to multi-path effects caused by reflections of the signals from man-made and/or natural constructions. For example, where there are n reflectors, a single aircraft can generate an indication of n aircraft. If there are actually two aircraft in the vicinity (with n reflectors) the number of aircraft presented to the controller could vary anywhere from 2n to 2n.sup.2, depending upon reflector geometry. If instead of two aircraft there are a hundred aircraft in an area with three large reflectors, the apparatus could indicate the presence of anywhere from 300 to 900 aircraft. Although the upper limit is highly unlikely, nevertheless, several hundred spurious replies will virtually prevent any effective control from the ground.
For example, FIG. 1 is a schematic illustration of a typical problem wherein a target locating system including an antenna 5 is attempting to locate the position of aircraft in its vicinity, such as aircraft 10 in the presence of a reflector 11. The apparatus generates interrogation signals, and typically these include a pulse coded signal comprising at least a pair of pulses, of approximately 30 nanoseconds duration. At least one of these pulses is omni-directionally transmitted and, at least another pulse, which is separated from the first pulse by about 3 to 20 microseconds, is directionally transmitted. The aircraft carried transponder is arranged to be suppressed for about 35 microseconds upon receipt of only one of the two pulses. The transponder, if it receives both the pulses, transmits a reply and then suppresses itself for about 100 microseconds. The target locating apparatus 5 assumes that the target transmitting a reply is at the same azimuth from which the reply was received. The target range is determined by the time delay between the interrogation signal and the reception of the reply. Transponder suppression will, for instance, prevent a reply if the directionally transmitted pulse traverses the path c-b in less than 35 microseconds. On the other hand, a reply will be generated if this pulse traverses the path in a longer time. The reply will be received either through the main antenna beam, if reflected, or through the antenna side or back lobe, if not reflected. If the reflector is a sufficiently efficient reflector or the reply power is sufficient to exceed the power threshold through the antenna back or side lobe, a spurious reply will be received.
Although it is clear that the spurious responses are generated by multi-path effects, the precise mechanism or mechanisms which produce the spurious replies has not been ascertained.
It is therefore an object of the invention to provide apparatus which can be used to modify the existing equipment so as to minimize or eliminate the spurious responses. It is another object of the present invention to provide such apparatus for modifying existing systems with the minimum disruption and expense. It is a further object of the invention to provide apparatus for modifying existing systems which is compatible with a wide variety of existing systems.
It is yet another object of the invention to provide a target locating apparatus which can be used in the vicinity of man-made and/or natural signal reflectors which at the same time selectively discriminates against false replies.